The invention relates generally to flat panel display manufacturing systems and more particularly to an apparatus for preparing polycrystalline silicon films on flat panel display substrates.
Thin film transistors (TFTs) used in liquid crystal displays (LCDs) or flat panel displays of the active matrix display type are fabricated on silicon films deposited on a transparent substrate. The most widely used substrate is glass. Amorphous silicon is readily deposited on glass. Amorphous silicon limits the quality of TFT that can be formed. If driver circuits and other components are to be formed on the display panel, as well as switches associated with each pixel, crystalline silicon is preferred.
Amorphous silicon can be crystallized to form crystalline silicon by solid-phase crystallization. Solid-phase crystallization is carried out by high temperature annealing. But, glass substrates cannot withstand the temperatures necessary to melt and crystallize silicon. Quartz substrates can withstand high temperature annealing, but quartz substrates are too expensive for most LCD applications.
Because glass deforms when exposed to temperatures above 600xc2x0 C., low-temperature crystallization (preferably below 550xc2x0 C.) is used for solid-phase processing of silicon on glass. The low-temperature process requires long anneal times (at least several hours). Such processing is inefficient and yields polycrystalline silicon TFTs that have relatively low field effect mobility and poor transfer characteristics. Polycrystalline silicon produced by solid-phase crystallization of as-deposited amorphous silicon on glass suffers due to small crystal size and a high density of intragrain defects in the crystalline structure.
Excimer laser annealing (ELA) has been actively investigated as an alternative to low-temperature solid-phase crystallization of amorphous silicon on glass. In excimer laser annealing, a high-energy pulsed laser directs laser radiation at selected regions of the target film, exposing the silicon to very high temperatures for short durations. Typically, each laser pulse covers only a small area (several millimeters in diameter) and the substrate or laser is stepped through an exposure pattern of overlapping exposures, as is known in the art. More powerful lasers with larger beam profiles are now available or are under active development, reducing the number of exposures required. Regardless of the number and pattern of exposures, ELA allows areas of amorphous film to be crystallized without damaging the underlying glass substrate.
The major advantages of ELA are the formation of polycrystalline grains with excellent structural quality and the ability to process selected areas of a display panel. Polycrystalline silicon produced on transparent substrates by ELA has electron mobility characteristics rivaling IC driver circuits currently mounted along the edges of the screen. Thus, it becomes possible to incorporate driver circuitry onto the substrate, simplifying manufacturing.
The most common problem that plagues ELA is the narrow process window associated with the development of large and uniform grain sizes. Surface roughness inherent to the process is also troublesome. Research has suggested that improvements in surface conditions, a reduction in defects, and increased crystal size are associated with low oxygen content ELA polycrystalline silicon films. Oxygen content can be controlled in several ways. The industry standard currently being used is to perform ELA in a high vacuum (10xe2x88x927 Torr), or somewhat less efficacious, in a rough vacuum (10xe2x88x923 Torr). Alternatively, ELA has been carried out in chambers filled with non-oxygen ambient gases such as He, Ar, or N with varying results. The association between oxygen content and polycrystalline silicon film quality is still being investigated.
A significant problem with prior art systems for reducing oxygen incorporation into polycrystalline silicon during ELA is the need for a process chamber to house the target substrate. When a process chamber (alternatively called: xe2x80x9cchamberxe2x80x9d, xe2x80x9cprocessing chamberxe2x80x9d, or xe2x80x9csubstrate isolation chamberxe2x80x9d) is used, the beam of the excimer laser must pass into the chamber through a quartz window. Vacuum chambers, in particular, are costly. Chambers for processing in non-air ambient at atmospheric pressure are somewhat simpler than vacuum chambers, but still have quartz windows. The quartz windows cost several thousand dollars and have only a limited life, lasting only days or weeks in volume production. The cost associated with a processing chamber is one reason ELA equipment without substrate isolation is still being manufactured, sold and used. The despite evidence that ELA performed in air ambient produces polycrystalline silicon with inferior mobility characteristics (and a higher oxygen content) compared with films annealed in non-air ambient.
It would be advantageous to be able to effectively control the amount of oxygen incorporated in ELA polycrystalline silicon films, keeping the oxygen content below a predetermined threshold, while minimizing the cost of production.
It would be advantageous to have ELA equipment that would reduce, or eliminate, oxygen from the target area without the need for an isolation chamber.
It would also be advantageous to improve the quality of ELA polycrystalline silicon films on flat panel display substrates by reducing oxygen incorporation with relatively simple changes to ELA equipment.
It would also be advantageous if existing equipment could be modified to reduce, or eliminate, oxygen from the target area without the need for an isolation chamber.
Accordingly, a laser annealing apparatus for forming polycrystalline silicon film on substrates using ELA is provided. The laser annealing apparatus of the present invention comprises a laser for directing a beam to a location on the surface of a semiconductor material. A nozzle, or plurality of nozzles, is positioned to direct a flow of gas over the location on the surface of the semiconductor material. The gas is preferably helium, neon, argon or nitrogen. The gas removes ambient air, especially oxygen, from the location on the surface of the semiconductor material. This allows the laser to anneal the semiconductor material in an atmosphere with reduced oxygen, or preferably no oxygen. The absence of oxygen allows the laser to produce a higher quality polycrystalline region within the semiconductor material.
The apparatus is adapted to be retrofit to existing ELA systems. It can be mount to a laser head or to a moveable base upon which the semiconductor material is placed.
The apparatus will preferably include an exhaust system to aid in removing the gas and ambient air.
In one preferred embodiment of the apparatus, a shroud is provided to surround a laser beam produced by the laser. The shroud incorporates an air path for the gas flow to the nozzle as well as an exhaust port. In a further embodiment, the shroud is partially sealed to the base supporting the semiconductor material forming an enclosure. The shroud is preferably flexible to allow for the movement of the base. The flexibility is preferrably provided by a flex region.